The products related to the RF identifying system in the related art have been utilized in guarded entrance systems, time card systems, inventory systems, delivery management systems, and animal identification systems. Recently, RF identifying technique has further been widely used in car key recognition and RF bar codes of the merchandise. Accordingly, it has great potential in commercial applications. However, the generation of the conventional RF identifying tag now in use is owing to the corresponding pattern of binary 1's and 0's in a data source that represent the variation of the load impedance of the controlling antenna. The efficiency of voltage-charging rectifying of the received RF electro-magnetic waves traveling through the dc source strongly depends on the fact whether the impedance of the semiconductor device and the read-only memory (ROM) substantially matches the load impedance of the antenna or not. In addition, the card reader can only accurately detect the variation of the load impedance in a distance shorter than ten centimeters (10 cm), it is thus very inconvenient as far as the operation of the users is concerned.
Please refer to FIG. 1 for description of the prior art. FIG. 1 is a somewhat schematic diagram illustrating a system of the conventional RF identifying technique. The schematic diagram in FIG. 1 is related to a resonator 32 that comprises an antenna 31 and is connected to a voltage-charging rectifier circuit 121. The voltage-charging rectifier circuit 121 is connected to a control logic 62 that is connected to a logic circuit for data storage 65. In the meantime, the voltage-charging rectifier circuit 121 is also connected to a time sequence modifying circuit 61 and a modulating circuit 64. The mentioned voltage-charging rectifier circuit 121 is composed of a rectifier diode 11 which is serially connected with a voltage limiter 12 and a capacitor 21. The mentioned voltage limiter 12 is a zener diode. Furthermore, the control logic 62 is connected to a demodulating circuit 63. The circuit device of the conventional RF identifying tag is thus completed.
As mentioned above, within the reading range of the card reader (not shown in the figure), generally ten centimeters, the antenna 31 in the circuit diagram of the RF identifying tag receives the RF signals f.sub.0 from the card reader, and the capacitor 21 is charged by the voltage-charging rectifier circuit 121 to the voltage level of a zener diode (used here as a voltage limiter 12) and play the role as a voltage supply V.sub.cc which provide the power supply for the entire system of the RF identifying tag. In the meantime, the time sequence modifying circuit 61 amplifies the received RF signals f.sub.0, and the received RF signals are then converted into time sequence modified signals with digital level and provide the time sequence modified signals for the entire system of the RF identifying tag.
Furthermore, the digital data from a register or a memory (not shown in the figure) are delivered by the logic circuit for data storage 65 into the modulating circuit 64 in accordance with the time sequence modified signals, and then are emitted from the antenna 31 into a card reader. The signals introduced from the card reader into the RF identifying tag are modulated RF signals received by the antenna 31 in the circuit device and then delivered into the demodulating circuit 63. The control logic 62 controls the operation of the whole circuit device for the RF identifying tag.
Referring to the prior art of RF identifying techniques, for example, in the system disclosed and claimed in the U.S. Pat. No. 4,786,907 entitled "Transponder Useful In A System For Identifying Objects" filed by Alfred R. Koelle on Nov. 22, 1988, when the modulated signals are emitted from an antenna, the signals directly control a switch and thus control the load impedance of the antenna. The technique is somewhat identical to the technique in the previously discussed conventional circuit device for RF identifying tag (as show in FIG. 1) in which the card reader can read the corresponding pattern of binary 1's and 0's in a data source in accordance with the magnitude variation of the electro-magnetic field due to the change in the load impedance of the antenna.
However, the system in the prior art has had certain drawbacks, mainly because the amplitude of the electric potential over the antenna coil induced by the change of the alternating magnetic field of the radio wave signals strongly depends on the fact whether the impedance of the semiconductor device and the read-only memory (ROM) substantially matches the load impedance of the antenna or not. Therefore, when the modulator of the identifying tag delivers the data, the switching on or switching off of the control switch 51 can cause the variation of the impedance and thus reduce the voltage-charging efficiency of the voltage-charging rectifier circuit 121; When the switch 51 is on, the positive terminal of the capacitor 21 is directly connected to the load of low impedance. Furthermore, the input signals of the time sequence modifying circuit 61 are converted to output signals of unstable amplitude also due to the switching on or switching off of the switch 51, and thus the designing complexity of the time sequence modifying circuit 61 increases. In addition, the change in load impedance due to the state changing of the control switch 51 only brings forth minimal variation in electromagnetic field, thus it is not easy for the card reader to detect the minimal variation. As a result, the method of using the control switch 51 to cause the change in load impedance for data delivery can only be used within a short distance, for instance, less than ten centimeters. Therefore, it is inconvenient for applications and suffers low accuracy.
Consequently, in order to overcome the discussed problems, the present invention provides a battery-free circuit device for RF identifying tag that has advantages such as a longer identifying distance and a larger detecting angle with better data reading accuracy and reliability than in the prior art.